For the realization of high frequency transistors and diodes as well as fast integrated circuits on substrate bodies consisting of semiconductor material, particularly consisting of III-V semiconductor material such as gallium arsenide, indium phosphide and the like, thin active layers of the semiconductor material are required which are situated on a high resistance substrate material. In conventional devices employing gallium arsenide, a thin active gallium arsenide layer is epitaxially grown on a semi-insulating gallium arsenide substrate body. The semi-insulating property of the gallium arsenide substrate body is achieved by means of chromium doping. Chromium doping can also be employed in substrates having indium phosphide to achieve the semi-insulating property.
A disadvantage inherent in the methods and resulting components described above is that the chromium therein is redistributed during the epitaxial growth step and/or during the thermal annealing process which is necessary to activate the implanted donor atoms. The chromium diffuses into the epitaxial layer as well as diffusing into those areas of maximum lattice deterioration in the implanted region. Moreover, the electrical properties of the active layer such as doping density, doping profile, mobility and the like are subject to undesired influences as a result of the chromium migration. The physical characteristics and operating parameters of components manufactured as described above become undefined as a result of the unpredictable and random effect of the chromium migration and thus result in broad operating tolerances.
A proposal to minimize the disadvantageous effects of the migration of the chromium into the active layer is to generate a so-called buffer layer between the substrate body and the layer which is epitaxially grown. The generation of such a buffer layer, however, involves signficant commercial outlay resulting in a significantly more expensive component.
It is also conceptually possible to prevent the chromium redistribution in the implanted layer by manufacturing the semi-insulating substrate without chromium doping, however, this concept would also involve a significantly increased outlay because great demands would be made of the crystal growth which would require extremely careful control.